Can I use Bluesky in Spanbox?

Yes. Bluesky has a web version, and Spanbox's whole model is that if an app has a web app, you can add it. So you sign into Bluesky inside Spanbox exactly like in a browser and it sits in your unified inbox with the rest, your feed and messages. It counts as one of your apps (3 free, unlimited on Pro).

You can even run more than one Bluesky account at once, each in its own isolated session, so a work and a personal Bluesky sit side by side without logging in and out.